1. Field of the Invention
The present invention relates to an optical cable for communication comprising at least one micromodule blocked with respect to the propagation of water, as well as to such a micromodule.
Transmission elements, in particular coated optical fibers, used in optical cables are typically housed, either individually or as a group, in buffering materials or elements. For instance, one or more optical fibers, e.g. arranged in a group, bundle or ribbon of optical fibers, may be housed in a tube or flexible sheath (hereinafter referred to as “retaining element”), which may be made, for example, of polymeric material. The optical fiber(s) together with its(their) retaining element is(are) generally referred to in the art as “optical unit”, “micromodule” or “module”. Usually, said retaining element has a thin thickness and is endowed with specific mechanical properties (in particular, elastic modulus, ultimate tensile strength and elongation at break) so as to allow an easy access to the optical fiber(s) in order to facilitate both the connection between the optical fiber(s) and an user equipment, and the interconnection between cables. Said retaining element is also generally referred to as “microsheath” or “minisheath”.
Moreover, said retaining element, usually, has an axial stiffness relatively low so as to slightly affect the fiber(s) attenuation due to mechanical stresses and strains which may occur during the thermal cycles to which said fiber(s) is(are) usually subjected.
An optical cable may contain a single micromodule or a plurality of micromodules. Said single micromodule or said plurality of micromodules are generally referred to as the optical core of the cable. The optical core is in turn typically inserted into a protecting sheath usually made of polymeric material.
Usually, each micromodule may comprise a bundle of optical fibers, typically in a number comprised between 2 and 12, housed in a retaining element as defined above.
The optical fibers are arranged parallel or according to an open helix pattern (or S-Z stranding) around the axis of the micromodule, i.e. the optical fibers are stranded around the axis of the micromodule in sections with a first direction of stranding (S-shaped) alternating with sections with an opposite direction of stranding (Z-shaped).
Within each micromodule, the optical fibers may be arranged with or without clearance between their outer envelope and the inner surface of the retaining element of the micromodule. If no clearance is left between the optical fibers and the retaining element, the micromodule is called tight, while in the opposite case, i.e. if there is a clearance between the optical fibers and the retaining element, the micromodule is called loose.
In the present description and in the following claims, a micromodule shall be indicated as loose when the optical fibers are mechanically decoupled from the retaining element. Consequently, for a suitable length of a micromodule of the loose type (e.g. 1 meter) it is possible to extract a single optical fiber independently of the other optical fibers. Usually, it is possible to operate as disclosed above when the inner diameter of the retaining element is at least 1% larger than the diameter of the smallest circle enveloping the optical fibers defined therein, otherwise the micromodule shall be indicated as tight.
As already reported above, the micromodule allows an easy access to the optical fiber(s) housed therein both at the free end of the micromodule and at an intermediate position of the micromodule by simply tearing and slipping off the retaining element. By exerting a moderate combined pressure and tensile strength with the fingers, in fact, a predetermined length of retaining element can be easily removed so as to access to the optical fiber(s) remained uncovered.
Consequently, the mechanical properties of the polymeric material constituting the retaining element indicate that, apart from the containing function of the latter, one of the main aims of providing micromodules in optical cables is that of grouping different bundles of optical fibers so as to allow an easy identification of the different bundles. Such identification may be attained, for example, by providing micromodules with respective retaining elements having different colors.
The arrangement of the optical fibers in micromodules allows to assemble a high number of optical fibers in a relatively small optical cable (e.g. a cable with up to 144 optical fibers may have an external diameter lower than or equal to about 13 mm or less; cable with a lower number of fibers may have a correspondingly lower diameter), which makes cables including micromodules particularly suitable for urban distribution networks.
In the present description and in the following claims, the expression “blocked with respect to the propagation of water” means that the propagation of water is prevented or limited both in the micromodule and in the optical cable containing the same, in the sense that both the micromodule and the optical cable containing the same pass the test according to method F5B provided by International Standard IEC 60794-1-2: further details regarding the above test will be given in the examples which follow. The propagation of water is mainly intended as a spreading along the longitudinal direction of the micromodule which results in a progressive filling thereof.
2. Prior Art
Optical cables for communication comprising at least one micromodule (usually a plurality of micromodules) blocked with respect to the propagation of water, are known. For example, U.S. Pat. No. 5,671,312 discloses optical cables comprising micromodules water-blocked by means of an oil having a viscosity comprised between 100 and 5000 mPa·s, such as for example a silicone oil. Each optical fiber is provided with such an oil by means of an applicator member such as a felt pad associated with an oil feed member and located upstream of the extrusion head used to extrude the retaining element around the optical fibers.
US patent application 2003/0168243 discloses an optical cable for telecommunication comprising micromodules water-blocked either by means of silicone or synthetic grease, oil or gel. The optical fibers are coated with such filling compounds before the optical fibers are passed through a die for extruding a thin retaining sheath clamping the optical fibers together. The use of swelling powder and/or swelling filaments is also envisaged. U.S. Pat. No. 5,751,880 discloses an optical unit for an optical fiber telecommunication cable, the unit comprising a tube of plastics material in which at least one optical fiber is loosely received, wherein the thickness of said tube is less than or equal to 0.5 mm, and wherein said material has a modulus of elasticity less than 1500 MPa at 20° C. and a stress/elongation curve without a yield point. Said tube could also contain a material providing sealing in the form of a gel which is not better defined.
Optical cables are also known wherein the optical fiber(s) are inserted in a tube, sometimes called “buffer tube”, which usually has a thickness higher than about 0.2 mm, typically of from about 0.3 mm to about 0.8 mm.
For example, International Patent Application WO 2004/034115 discloses buffer tubes, core tubes or slotted core fiber optic cable components, which are made of an extrudable blend of highly crystalline polypropylene and an impact modifying polymer. As disclosed in the above-mentioned patent application, said buffer tubes, which are modeled as having a 3.3 mm outside diameter and a 0.76 mm wall thickness, are typically filled with an optic cable hydrocarbon-based grease incorporating hydrocarbon oils surrounding the fibers and eliminating air space. The above-mentioned grease (also referred to as “gel”) is said to provide a barrier against water penetration, which is detrimental to the optic transmission performance.
U.S. Pat. No. 5,911,023 discloses optical cable components such as buffer tubes, filler rods or jackets, made of a thermoplastic polyolefin, preferably propylene or ethylene homopolymer, a propylene-ethylene copolymer, or a terpolymer including propylene and ethylene, characterized by a high melt flow index. The use of said material having a high melt flow index results in a substantial improvement in buffer tube crystallinity and crystallization rates, improved buffer tube crush resistance, reduced post extrusion shrinkage, improved gel compatibility, and improved excess fiber length control. The gel in the buffer tube is said to be a thixotropic, water blockable gel such as mineral gels, or petroleum gels.